1. Field of the Invention
The present invention relates to a charging apparatus. More particularly, the present invention relates to an improvement in a charging apparatus for supplying a high frequency charging current to a rechargeable battery by the use of an inverter including a semiconductor switching element.
2. Description of the Prior Art
For example, rechargeable batteries are housed in a variety of small-sized, electric appliances including electric shavers. These electric appliances housing such rechargeable batteries are very convenient to use due to the feasibility of recharging the rechargeable battery if they are plugged into a commercial power supply, for example. However, a line voltage of the commercial power supply differs in various countries. An adapter which is compatible with different line voltages in these different countries is therefore necessary in order to recharge the rechargeable battery from these different line voltages. Such an adapter, however, is bulky and inconvenient to carry during travel, etc. Furthermore, there is the possibility of causing fire or other troubles in the event that an electric appliance for 100 volts use is inadvertently plugged into a commercial power line carrying for example 240 volts. In recent years, several approaches have been proposed in an attempt to develop a charger for use with a rechargeable battery which is adaptable to different line voltages without the need for the above mentioned adapter. A typical example of those approaches is disclosed in the U.S. Pat. No. 3,599,071 issued Aug. 10, 1971 to Robert Henri Lapuyade, for example. In U.S. Pat. No. 3,599,071, there is provided a transformer supplied with the commercial power supply. A plurality of voltage selection taps are connected to a primary coil of the transformer, one of which is manually selectable depending on the power line voltage available in the country where the user is to use the charger. However, the transformer as proposed in U.S. Pat. No. 3,599,071 is inferior in efficiency due to its iron and copper losses. Since a charging current having the commercial power frequency is supplied in the apparatus disclosed in U.S. Pat. No. 3,599,071, the transformer should be large in size and an electric appliance housing those components also should be large in size. Moreover, since the taps on the transformer are manually selectable, a failure or a mistake to select a manual switch results in various troubles as set forth above. Due to provision of the manual switch and the voltage selection taps on the transformer a further increase in size of electric appliances with such a built-in charger is unavoidable.
Chargers using a transistor inverter rather than the transformer as taught by the above referenced U.S. Pat. No. 3,599,071 have recently been proposed and deemed as more practical. FIG. 1 is a circuit diagram of an example of the prior art inverter type chargers which is relevant to the background of the present invention. An alternating current voltage supply 1 such as the commercial power source is connected to a rectifier 2. Supplied from the rectifier 2 to an inverter 3 is a pulsating output which is half-wave rectified from the alternating current voltage supply. The inverter 3 converts the output of the rectifier 2 into a high frequency voltage which in turn is rectified to provide a charging current for a rechargeable battery 9 such as a Ni-Cd battery. This inverter type of charger is well known in U.S. Pat. No. 3,869,657 issued Mar. 4, 1975 to Shoki et al and assigned to the assignee of the present application and so forth. To give a better understanding of the present invention, the circuit of FIG. 1 will now be described in more detail.
A resistor 21 included in the rectifier 2 is of a self-burning type which serves to shut off a primary circuit when a secondary circuit is out of order. An inductor 22 and a capacitor 24 included in the rectifier 2 form a noise filter. The inverter 3 includes a primary coil 41 of an oscillation transformer. Interposed in a current path or a primary current path of the power converter 4 is a switching circuit 5 which comprises a switching transistor 51 with its base connected to an accelerator circuit 7 which is enabled with a trigger signal from a trigger signal generator 6. The trigger signal generator 6 comprises a series connection of a resistor 61 and a parallel circuit including a resistor 62 and a capacitor 63. The trigger signal is derived from the junction of the series connection and supplied to the base of the transistor 51 through a base feedback coil 71 included in the accelerator circuit 7. The base feedback coil 71 is included in the oscillation transformer with magnetic coupling with its primary coil 41. The primary coil 41 of the oscillation transformer is also magnetically coupled with its secondary coil 82 included in a secondary output rectifier 8. This secondary output rectifier 8 includes a diode 81 for rectification of the output from the secondary coil 82. The output of the diode 81 is supplied as the charging current to the rechargeable battery 9.
The above described charger operates in the following manner. The alternating current output from the alternating current voltage supply 1 is half-wave rectified by means of the rectifying diode 23 in the rectifier 2 and then supplied in the form of a pulsating waveform to the trigger signal generator 6 included in the inverter 3. A current flows through the resistor 61 to charge the capacitor 63. Depending on the charging voltage on the capacitor 63 the base-emitter voltage (V.sub.BE) of the switching transistor 51 increases. As soon as the base-emitter voltage (V.sub.BE) is in excess of the operating threshold level of the transistor 51, the transistor 51 starts turning on to permit a primary current I1 shown in FIG. 2A to flow through the collector-to-emitter path of the transistor 51 and in other words the primary coil 41 of the oscillation transformer. The primary current I1 flowing the primary coil 41 results in developing an induced voltage on the base feedback coil 71. The base feedback coil 71 operates in a direction to increase a base-emitter current (I.sub.BE) as depicted in FIG. 2B and to effect positive feedback to the switching transistor 51. Under these circumstances the switching transistor 51 becomes completely conductive in a very brief period of time. If the current I1 flowing through the primary coil 41 increases as shown in FIG. 2A, then the induced current will be no longer supplied to the base feedback coil 71 in response to the primary coil 41 being magnetically saturated or a collector-emitter current (I.sub.CE) of the transistor 51 being saturated. Since a current supply to the base is prohibited in this manner, the transistor 51 becomes non-conductive. Once the switching transistor 51 has been turned off a voltage is induced in the opposite direction on the primary coil 41, reversing the direction of the current flowing through the base feedback coil 71 as seen from FIG. 2B. The switching transistor 51 is thus reversely biased to accelerate the turning off thereof. When the reverse voltage is induced on the base feedback coil 71 as depicted in FIG. 2B, there is developed a secondary current I2 as shown in FIG. 2C in the secondary coil 82 in the direction of conducting the diode 81. The secondary current I2 is supplied as the charging current to the rechargeable battery 9 via the diode 81.
The series connection of a capacitor 42 and a resistor 43 included in the power converter 4 serves to absorb a spike voltage at the primary coil 41 and suppress noise. A semi-fixed resistor 52 connected to the emitter of the transistor 51 in the switching circuit 5 is to limit the primary current I1. In other words, with adjustments of the semi-fixed resistor 52, the primary current I1 may be varied. The resistor 61 in the trigger signal generator 6 functions as a starting resistor which turns the switching transistor 51 on under an initial state, while the capacitor 63 serves to accelerate the turning on of the switching transistor 51. The resistor 62 connected to the capacitor 63 functions as a discharging resistor for the capacitor 63. The charge on the capacitor 63 is furnished to the base feedback coil 71 through the resistor 62, thus accelerating the turning on of the switching transistor 51.
The inverter type charger as described above has advantages over U.S. Pat. No. 3,559,071 in that it eliminates the need for a large-sized transformer and reduces space requirements of electric appliances to a minimum. The inverter type charger as shown in FIG. 1 however has problems as follows. If the commercial power supply of for example 240 volts, higher than the rated voltage of for example of 100 volts, is connected as the alternating current voltage supply, then there is developed an increase in the input voltage V.sub.IN supplied to the inverter 3 via the rectifier 2. This further leads to an increase in the charging current supplied from the secondary output rectifier 8 to the rechargeable battery 9 and adds a possibility of the overcharging of the rechargeable battery 9. Furthermore, if the input voltage V.sub.IN is too high, the oscillation transformer becomes saturated, increasing iron loss and calorific value of a core about which the coils 41, 71 and 82 are commonly wound. Such heat gives the user of electric appliances uneasy and disagreeable impression. In addition, in the light of the high voltage V.sub.IN semiconductor elements such as diodes and transistors with high ratings should be selected at the stage of circuit design with an accompanying increase in cost.
As pointed out previously, no effective approach has been proposed for the inverter system charger heretofore which provides a proper charging current in response to different supply voltages from alternating current voltage supplies such as the commercial power supplies in different countries and offers many advantages as compared with the present invention.